1. Field of the Invention
The invention relates generally to perimeter sewing systems, and more particularly to perimeter sewing systems for vehicle air bags.
2. Description of the Prior Art
The increasing popularity of air bags in motor vehicles has brought about a dramatic increase in the manufacturing demand for such products. At the same time that the popularity of such products has soared, there has emerged a need to devise manufacturing methods which reduce the manufacturing time and cost. One of the major manufacturing operations associated with fabricating air bags involves a perimeter stitching operation which attaches a front and rear fabric panel together to form the basic air bag. This perimeter stitching operation has created some difficulties because of certain manufacturing specifications which must be adhered to, as well as the nature of the fabric materials involved.
With respect to the manufacturing specifications, it is commonly required that air bags be perimeter stitched to very exacting requirements. These requirements include such things as the number of stitches per inch, the distance of the stitches from the perimeter edge of the materials being sewn, and overstitch requirements (which relate to perimeter stitching slightly more than 360 degrees around the bag perimeter). The types of materials involved have led to problems on the part of bag fabricators with meeting such requirements, in part, because the materials tend to have a surface which is somewhat slippery, with the result that the sewing process can sometimes lead to bunching of the fabric, misalignment of fabric panels, and undesirable pleating.
In order to solve the various problems presented by the air bag fabrication process, perimeter sewing systems have been developed which make use of rotating platters on which a pair of fabric panels are placed and then stitched together around their perimeter. Usually, a pair of platters is used, with one lower platter forming a base or support, and a second upper platter compressing against the lower platter in order to position the air bag panels therebetween. In order to meet the stringent manufacturing specifications associated with air bags, such perimeter sewing systems have typically used a plurality of stepper motors for separately driving the platters and the sewing machine. The stepper motors have been electronically synchronized so that the rotation of the platter and the sewing machine stitching action result in a perimeter stitching system which meets the requirements for number of stitches per inch, overstitching, etc.
While such prior art systems have met with some success, they have been found to present severe maintenance problems due to their rather complex nature. Also, because such systems have been digitally controlled and involve the use of multiple stepper motors operating at relatively high speeds, it has proved difficult to fully achieve the necessary degree of synchronization. For example, the process of synchronization for a given size air bag has necessitated that technicians manually input a set of synchronization control parameters and then perform test runs to determine whether proper synchronization has been achieved. Thus it has proved time consuming and, in some cases, nearly impossible to achieve proper synchronization using such systems.
Recently, the problem of synchronization has become of greater difficulty due to the use of certain new types of air bag materials such as 420 denier uncoated, 630 denier uncoated, 420 silicone nylon and 420 neoprene nylon offered by Reeves International and Milliken. Although the precise reason is not entirely clear, such fabrics have proved extremely difficult to use with perimeter stitching systems of the prior art. Such attempts have resulted in bunching and pleating of fabric as well as an inability to accurately control the number of stitches per inch for various size bags.
It would be desirable to provide a perimeter stitching system which avoids the maintenance problems which have plagued multiple step motor systems of the prior art which are electronically synchronized. It would be further desirable to provide a perimeter stitching system which allows continuous "on the fly" adjustment of synchronization between sewing machine stitch rate and platter rotation.